1. Field of the Invention
The present invention relates to a communications control system, a communications control method, and a mobile station and a base station for use therein, for controlling packet communications between the base station and the mobile station such as CDMA packet communications.
2. Description of the Related Art
A W-CDMA system, a third-generation mobile communications control system, adopts a method of adaptively switching between a shared channel and a dedicated channel in accordance with traffic.
On a RACH (random access channel), a type of shared channel, a packet is separated into a preamble signal (preamble portion) and packet data (data portion) for transmission. A mobile station transmits, before transmitting the packet data, the preamble signal several times, gradually increasing in predetermined steps the transmission power from a small transmission power level until receiving an acquisition indicator AI from a base station.
Upon detecting the preamble signal, the base station transmits an acquisition indicator AI to the mobile station on the RACH. When receiving the acquisition indicator AI, the mobile station starts transmitting the packet data at a transmission power level determined based on the transmission power level of the preamble signal having been transmitted immediately before. This method is called “power ramping” which allows transmission power control for each packet.
When the amount of packets stored in a transmission buffer becomes equal to or more than a predetermined threshold, a DCH (dedicated control channel) is established between the mobile station and the base station as in circuit switching communications, and resources for packet communications between the mobile station and the base station are exclusively allocated.
With this, a control channel associated with the dedicated control channel DCH performs packet data transmission power control on a regular basis, eliminating the need for power ramping.
On the other hand, a “reservation method” is known as a technique for efficiently using an RACH. In the reservation method, a mobile station transmits a very short reservation signal to a base station before transmitting a packet, and the base station controls the packet transmission by the mobile station to avoid packet data collision, in view of its resource allocation conditions.
For the reservation method, various systems such as Reservation ALOHA, SRMA (split-channel reservation multiple access), PRMA (packet reservation multiple access), DQRUMA (distributed queuing request update multiple access) have been proposed for practical use.
For the reservation method, a method of transmitting a reservation signal on an RACH such as a Slotted ALOHA system, a method of transmitting a reservation signal in minislots which are fixedly allocated to each mobile station (e.g., CSMA: global scheduling multiple access), and a method of selectively using the above two methods in accordance with the type of traffic (PODA: priority-oriented demand assignment) are known.
The use of the reservation method for a base station to control packet transmission by mobile stations enables solving a so-called “hidden terminal problem” such as signal collision between mobile terminals each out of the other's view.
Further, specification of the priority or the like of packets in reservation signals allows a base station to schedule transmission of packets in accordance with the priority and also to deal with QoS control in uplink packet transmission.
CDMA communications have a near-far problem and require packet transmission power control to obtain sufficient channel efficiency in cellular environments.
Fading, shadowing, interference, moving of mobile stations and the like dynamically vary transmission characteristics in a radio link between mobile stations and a base station. Packet communications in which a burst of traffic is produced thus require transmission power control on a packet basis such as the above-described power ramping.
A conventional communications control method using power ramping, however, has a problem that it requires adjustment of transmission power using a preamble signal for each packet, resulting in increased overhead.
The communications control method further has a problem that fixing the packet data transmission power at the start of transmission of packet data prevents continuous transmission of packets containing lengthy packet data.
The communications control method still further has a problem that it is difficult for it to conduct QoS control according to packet types.
On the other hand, a communications control method utilizing the above-described reservation method has a problem that it does not give consideration to packet data transmission power control.
The communications control method further has a problem that higher the packet transmission speed, larger the decrease in efficiency due to delay in response from a base station to a reservation signal.
The communications control method still further has a problem that when reservation signals are transmitted over a RACH, packet collisions can cause delay in reception of the reservation signals, causing quality degradation especially in real-time communications such as voice communications which allow short delay time.